1. Field of the Invention
The present invention relates to a fuel injector for injecting fuel into a combustion chamber of an internal combustion engine.
2. Description of the Prior Art
The publication EP 1 612 403 A1 has disclosed a fuel injector of this generic type that includes a valve piston, which is guided so that it is able to execute a stroke motion in an injector body and cooperates with the nozzle needle in its stroke motion. According to another design of fuel injectors likewise of this generic type, the nozzle needle protrudes directly into the region of the control valve. The valve piston and the nozzle needle delimit a control chamber that can be acted on with high fuel pressure. If the control chamber is acted on with high fuel pressure, then the valve piston and nozzle needle are moved along the stroke axis toward the injection openings in the lower region of the fuel injector so that the injection openings are closed. If the pressure in control chamber is relieved, then the valve piston and valve needle lift away from the injection openings by executing a movement along the stroke axis. Consequently, the movement of the valve piston and nozzle needle can be controlled by means of the pressure in the control chamber. The fuel injector also includes a valve component that transitions into a guide section that is cylindrically embodied and extends into the guide bore of a valve needle. Consequently, the valve needle is guided on the guide section and is able to assume an open position and a closed position by moving along the stroke axis.
In order to vent the control chamber, a conduit system is provided, which is composed of a riser bore and at least one transverse bore. These bores vent the control chamber into an annular chamber that is provided in the form of a constriction in the circumference region of the guide section. If the valve needle is situated in a lower vertical position along the stroke axis, then the annular chamber is closed so that the pressure in the control chamber remains at the high fuel pressure level. If the valve needle is lifted, then this allows the compressed fuel to flow out of the annular chamber into a discharge chamber so that the pressure in the control chamber decreases. Adjoining the control chamber in the transition to the riser bore, an output throttle is provided in order to limit the rate of the pressure decrease and therefore the stroke speed of the valve needle.
With such an arrangement of an output throttle, the problem arises that a large dead volume or waste volume prevails in the region of the riser bore, the transverse bores, and the annular chamber. Since the large opening cross section of the control valve causes a cavitation to rapidly occur in the flow in the region of the output throttle or downstream of it during an opening stroke of the valve needle with a large armature stroke (>20 μm), the volume in which this cavitation occurs is filled up with vapor. After the closing of the control valve, the volume must be refilled with fuel in opposition to the gas pressure; the pressure is increased up to the high fuel pressure (rail pressure). Only then does the valve piston cause the nozzle needle to close the injection openings again. The greater the proportion of vapor inside the waste volume, the longer the closing procedure of the nozzle needle takes; this closing procedure is subject to correspondingly large variances. This impairs the stability of the injections and the stroke-to-stroke variance increases from injection to injection. There is also a rise in the possible interval before a subsequent injection, thus decreasing the efficiency of multiple injections.
The object of the present invention, therefore, is to create a fuel injector in which it is possible to reduce the waste volume downstream of the output throttle and consequently to reduce the variance from injection to injection. Another object of the present invention is to reduce the time required to build up the high fuel pressure inside the control chamber.